This invention relates to a method for preparing catalysts useful in the manufacture of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly, it is directed to the preparation of catalysts suitable for producing maleic anhydride from saturated hydrocarbons.
Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
The prior art discloses a number of catalysts used in the conversion of organic feed stocks to maleic anhydride. As an example, U.S. Pat. No. 2,773,836 discloses phosphorus-vanadium-oxygen catalysts for the conversion of olefins to maleic anhydride. The catalysts had a weight ratio of V.sub.2 O.sub.5 to P.sub.2 O.sub.5 of 3:2 to 1:2, and were prepared by adding a vanadium compound to phosphoric acid, optionally adding a carrier to the solution, removing the excess liquid by evaporation, drying the remaining material at 200.degree. - 400.degree. F., grinding the resulting solids, and heating to 700.degree. -1100.degree.F. for several hours. U.S. Pat. No. 3,156,707 also discloses a similar method for preparing phosphorus-vanadium-oxygen catalysts for the conversion of olefins to maleic anhydride. The vanadium in these catalysts was reduced to an average valence in the range of 2.5 to 4.6 using an acid such as hydrochloric acid or oxalic acid during the preparatory steps, but the materials were calcined at elevated temperatures in air before use as catalysts.
Of particular interest is U.S. Pat. No. 3,293,268 which teaches a process of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under controlled temperature conditions and in the presence of phosphorus-vanadium-oxygen catalysts. One method taught in that patent for preparing catalysts comprised reacting phosphoric acid with a vanadium compound in aqueous hydrochloric acid solution, recovering the remaining solids containing tetravalent vanadium by evaporating the solution to dryness, and then heating the solids to about 450.degree.C. for two hours. The solids were ground to pass a 20 mesh screen and pelleted to form tablets. The tablets were then charged to a fixed catalyst bed in a test reactor at room temperature and the reactor heated for 16 hours at 400.degree.C. Thereafter, a 0.5 volume percent butane in air mixture was passed through the catalyst in a fixed tube reactor at temperatures above 400.degree.C. to form maleic anhydride before the catalyst was used in successive runs to convert butane to maleic anhydride.
All of these teachings in the prior art fail in one or more ways to achieve the results obtained by the use of the present invention. When phosphorus-vanadium-oxygen catalysts as prepared by the prior art, are calcined at elevated temperatures, the vanadium is oxidized to the pentavalent state. If the calcination is not conducted at too high a temperature, or over too long a period of time, the catalysts can be conditioned by passing a saturated hydrocarbon in air mixture over the catalysts at elevated temperatures to reduce the vanadium before the saturated hydrocarbon is converted to maleic anhydride in high yields. Sometimes, calcination at elevated temperatures over too long a time converts the vanadium to the pentavalent state, and the catalyst is not suitable for the conversion of saturated hydrocarbons to maleic anhydride. On the other hand, the present invention provides the art with a process of calcining a phosphorus-vanadium-oxygen catalyst to carefully control the level of vanadium in the pentavalent state. Thus, the catalysts prepared by the process of the present invention can be used for the conversion of saturated hydrocarbons to maleic anhydride without an activation step that requires the use of valuable production facilities for calcination or activation, rather than for the conversion of saturated hydrocarbons to maleic anhydride.